I went bivvying for the first time on Saturday night. Had "budget" kit, and it worked really well. Wasn't cold at all.
Bivvy bag: that Karrimor one. Can't find it on Sports Direct any more which is a shame. No condensation issues, but it wasn't a damp night.
Sleeping bag: Forclaz Trek 900 from Decathlon.
Mattress: Air Structure 700 (short version) - this kept deflating overnight, so today I'm going to see if I can find a leak. Will be disappointed if it's fucked, because it's a really nice bit of kit. Didn't stop me sleeping quite well, however.
Other shit: thermal leggings from Lidl, silk bag liner (didn't need), cheapo Decathlon down jacket (used as a pillow).
All in all quite a success.